Field of Invention
The present invention relates to a power module. More particularly, the present invention relates to a power module used in a POWER converter.
Description of Related Art
High efficiency and high power density has been the industry's requirements for power converters. High efficiency means less energy consumption, and energy saving helps to reduce carbon and protect environment. High power density stands for small size, lightweight and less space requirement, thereby reducing costs.
The energy consumption of the power converter is mainly composed of an on-state loss and a switch loss, especially the switch loss of an active device. The switch loss is more significantly affected by a working frequency. The power converter, especially the switch power converter, has the working frequency usually higher than 20 kHz in order to decrease audio noise. The selection of an actual working frequency of the power converter is more significantly affected by an inactive device, especially a magnetic element. If the magnetic element has a small size, a high frequency is usually needed to decrease the magnetic flux density of the magnetic element in order to achieve reliable work, thus inducing a high switch loss. Alternatively, the wire diameter of the wire set can be decreased and the number of loops in the magnetic element can be increased to increase the on-state loss.
On the contrary, if the magnetic element has a large size, the working frequency can be lowered under the precondition of assuring the reliable work, thus decreasing the switch loss. Also, the wire diameter of the wire set can be increased or the number of loops in the magnetic element may be decreased to decrease the on-state loss, thus decreasing the overall loss and obtaining high efficiency.
Therefore, one of the key factors of obtaining the high power density or the high efficiency is to enhance the space availability inside the power converter. As the space availability gets higher, the larger space is left for the inactive device, such as the magnetic element, a capacitor or the like, in which the inactive device is very important to the power converting efficiency. Thus, the large-size inactive element can be easily used to increase the power efficiency. Also, the total power of the power source can be increased by using the large-size inactive device, so that the power density of the power converter can be enhanced. Thus, for the high power space availability, the high efficiency can be achieved more easily under the specific power density, or the high power density can be achieved more easily under the specific efficiency, and it is possible to possess both the high power density and the high efficiency concurrently.
In addition, a semiconductor device is one of the important factors for determining the efficiency of the power converter. However, the use of the semiconductor device tends to unavoidably need to use additional materials, such as a packaging material for protecting the semiconductor device, a heat sink for heat dissipating, a fixture for fixing the semiconductor device, and the like. As the ratio of these materials inside the power converter gets greater, the internal space availability of the power converter gets worse. As a result, the ratio of the space, occupied by the power semiconductor device, to the total size of the power converter gets larger and larger, and gets more and more emphasized. In order to enhance the performance of the power converter, the space availability of the power converter has to be continuously enhanced. The package space availability of the semiconductor device becomes a bottleneck.
For an integrated power module (IPM), many semiconductor devices (e.g. a power device, a controlling device, a driving device) are integrated within a device package for the enhancement of the space availability within the device package. The power module has the advantages including use convenience and long average operation time without faults, etc., and is widely applied to various occasions. Because the power module has many power chips integrated together, a lot of heat is generated and distributed in many points of the power module. The thermal management thereof thus becomes very important. There are many existing arts for improving the heat dissipating ability of the IPM.
Referring to FIG. 1a, FIG. 1a is a schematic diagram showing a conventional power module 100a. As shown in FIG. 1a, the power module 100a includes a first power device 11, a second power device 12, a substrate 13, a bonding wire 14, a lead frame 15, and a molding material 16. The substrate 13 is a direct bonded copper (DBC) ceramic substrate, which is made from a copper layer 131 with good thermal conductivity and a ceramic substrate 132 with high insulation. A circuit pattern is formed on the DCB ceramic substrate, and then the respective power devices 11 and 12 are assembled with the DBC ceramic substrate. Then, with respect to parts of the electrodes on the first power device 11 and the second power device 12, the bonding wire 14 is adopted to accomplish the electrical connections between the front-side electrodes of the first/the second power devices 11, 12 and the DBC substrate and the lead frame 15. Thereafter, a molding material 16 is injected to enclose the areas desired to be protected, thus achieving dustproof, moisture-proof and insulation functions.
However, because all of the power devices have to be mounted on the DBC ceramic substrate, the DBC ceramic substrate with a larger area is required. However, the DBC ceramic substrate is relatively expensive, thus increasing the cost of the entire package module. In addition, the DBC ceramic substrate 132 is generally formed from aluminum oxide of which the coefficient of heat conductivity is equal to about 24 W/m·K, which is a great improvement with respect to the molding material (of which the coefficient of heat conductivity is generally lower than 1 W/m·K). However, the heat conducive property of aluminum oxide is still worse than that of metal (e.g., the coefficient of heat conductivity of copper is equal to about 400 W/m·K), so that the transversal heat diffusion ability of the DBC ceramic substrate is not good enough, and the poor thermal uniformity thereof tends to occur. Thus, in the conventional method, additional heat sink is added to expand the heat dissipating area and improving the thermal uniformity.
Referring to FIG. 1b, FIG. 1b is a schematic diagram showing another conventional power module 100b. Similar to the first power module 100a shown in FIG. 1a, the power device 100b includes the first power device 11, the second power device 12, and the substrate 13, in which the substrate 13 is a DBC ceramic substrate, and the first power device 11 and the second power device 12 are disposed on the substrate 13. Another side of the substrate 13 is disposed on the heat-dissipating unit 17 (e.g. a heat sink). The heat sink can be formed from good thermo-conductive materials, such as copper, aluminum, graphite or the like, so that the thermal uniformity performance of the power module 100b can be greatly increased.
Because the DBC ceramic substrate has high stress withstand capacity, a thicker molding material is required to ensure the overall insulation and stress withstand capacities. Because the heat dissipating ability of the DBC ceramic substrate is better, the DBC ceramic substrate is often designed for the application with a higher thermal density, and screws are adopted to fix the additional heat sinks. Because of high stress withstand packaging, the corresponding screws holes also need to be designed for stress withstanding, and thus occupies larger actual space. For example, a screw hole with a 3 mm hole diameter generally occupies an area of which the diameter is greater than 5 mm for meeting the stress withstanding requirements of the power module, thus lowering the space availability of the power module.
Furthermore, the power device 100b also includes a controlling/driving device 18. Because the controlling device and the driving device have a low energy consumption, and are relatively sensitive to temperature, they are usually disposed on the heat-dissipating unit 17 through a thermal insulating layer 19 (such as a printed circuit board (PCB), a molding material or the like). The thermal conductive insulating layer 19 can be formed by adhering, filling, or coating on the surface. Thereafter, the wire bonding is performed to accomplish the electrical connections among the first power device 11, the second power device 12, the controlling/driving device 18, the substrate 13 and the lead frame 15, and then the molding material 16 is injected to complete the fabrication of the packaging of the power module 100b. Accordingly, the device with low power consumption and being sensitive to heat can be integrated into the power module with less high-temperature influence from the power device, thereby improving the space availability of the power module.
Although the space availability of the power module can be enhanced by disposing the controlling device or driving device on the heat sink through thermal insulating layer, the aforementioned problems of the DBC ceramic substrate still cannot be overcome. Besides, the shell of the power module (not shown in FIG. 1b) is generally designed to be insulated to simplify the installation and selection of the heat sink. Hence, even if the material of the shell is a good electrical conductor (e.g., copper), the shell is still designed to be electrically insulated. Thus, the metal material (such as copper) in the power module is merely used to provide one single function of electrical or thermal conduction, and its electrically and thermally conductive properties are not utilized simultaneously, thus not fully utilizing the features of the material.
In sum, the conventional power modules still have various problems such as poor heat dissipating performance, material wastage, the difficulty of reliability design, not fully utilized electrical performance, the over design caused by over-emphasis on generality, and poor economic performance, etc. More particularly, the conventional power modules have insufficient space availability, and their applications in high power density or high efficiency occasions are thus restricted. In order to further increase the power density or converting efficiency of the power converter, there is a need to develop a power module with high space availability and reasonable cost.